Relationship of Web Characteristics and Body Measures of Leucauge decorata (Araneae: Tetragnathidae)

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1 Pakistan J. Zool., vol. 42(3), pp , Relationship of Web Characteristics and Body Measures of Leucauge decorata (Araneae: Tetragnathidae) Hafiz Muhammad Tahir*, Abida Butt and Imtiaz Alam Department of Zoology University of the Punjab, Lahore. Abstract.- Present study was conducted to investigate the relationships between web characteristics and body measures of Leucauge decorata (Araneae: Tetragnathidae). For this purpose a rice field (800 m 2 ) in village Kirka located in Lahore district was selected. The data of spider s webs and prey was collected in September 2007 and Most of the L. decorata constructed inclined webs at the height that ranged from 40 cm to 123 cm above ground. Spiders recorded from the higher webs ( cm) were larger and heavier than the spiders recorded from the lower webs (40-80 cm). Time required to complete a web was 1.00 ± 0.21 hour. The average diameter of the web was 25 ± 6.8 cm. Principal component analysis (PCA) did not separate the collected spiders on the basis of capture area (web character) and carapace width (body measure). Most of the prey items recorded from the webs belonged to Diptera, Homoptera and Lepidoptera. Prey types and their number collected from the webs of different heights varied significantly. Capture area of L. decorata web showed a positive correlation with carapace width and body weight. Web characteristics (i.e., number of radii, number of spirals and mesh height) were not correlated with any of the body measures (i.e., carapace width, total length and wet weight). For prey capture L. decorata construct webs of different sizes and at different heights but always maintain the basic web architecture (i.e., number of radii, number of spirals and mesh height). Key words: Web characteristics, body measures, orb web spider. INTRODUCTION Orb-weaving spiders are highly diverse, abundant in the fields and important regulator of pest populations in agro-ecosystems (Levi, 1981; Nyffeler and Benz, 1989; Aiken and Coyle, 2000). To optimize behavioural efficiency and to reduce energetic cost and time, a predator has to make several choices in the fields (Alcock, 1993). These choices are: where to construct a web, where to eat, how much time to dedicate to eating and what types of prey to select for capture and consumption (Henaut et al., 2006). Orb-web spiders do not usually compete for food or space (Wise, 1993), as different species use different predatory strategies (Henaut et al., 2006). One such strategy that reduces competition is construction of webs, which differ in size, location and general architecture (Rypstra, 1982; Eberhard, 1986; 1990; Sherman, 1994). Webs of spiders act as a filter and trap a large diversity of insects (Henaut et al., 2006). But variation in the design of the webs influence directly the length, kind and number of * Corresponding author: hafiztahirpk1@yahoo.com /2010/ $ 8.00/0 Copyright 2010 Zoological Society of Pakistan. prey entangled (Miyashita and Shinkai, 1995). Leucauge decorata (Blackwell, 1864) is a common orb-web spider in ago-ecosystems of Punjab, Pakistan (Tahir and Butt, 2008). In order to understand the ecological role of this species in agro-ecosystems it is necessary to have complete information about the habitat preferences, web construction behaviour, high activity period, food preferences and reproductive biology. Present study is aimed to record the web characteristics and type of prey caught in webs of L. decorata, relationship of prey and predator size and also the relationship (if any) between different body measures (i.e., total length, carapace width and wet weight) of L. decorata to its various web characteristics (i.e., mesh height, number of spirals, number of radii, capture area). This study will help to evaluate the role L. decorata as biological control agent for pests in agro-ecosystems. MATERIALS AND METHODS Study site The study site was a basmati rice field (800 m 2 ) in village Kirka located in district Lahore (latitude N, longitude E) about 35 km east from University of the Punjab, Lahore. The

2 262 H.M. TAHIR ET AL. experiment was performed in September 2007 and At the time of the experiment the average height of the plants in the fields was 133±13cm (n=50). During the study the daily temperature ranged from 27 ± 4 C (at night) to 37±4 C (during the day). The relative humidity was highly variable (68-88%) due to rainy season. Characteristics of the web Web characteristics were measured directly in the field after removing corresponding spider from the web (n = 115; 65 in 2007 and 50 in 2008)). For this purpose study was conducted early in the morning ( ) when the webs were clean and undamaged. Only the webs of adult specimens were used for the study. Before recording the web data, each web was sprayed with a fine mist of water and cornstarch using Knapsack hand sprayer (THS ) to improve the resolution. Web characteristics recorded were: web diameter, web height from the ground surface, capture area, mesh size, number of radii and number of spirals. Time required to complete a web was also noted (n = 25 each year). Spider characteristics The occupant of each web was collected in a glass tube (5 cm long and 2 cm wide, mouth covered by mesh cloth) and brought to the laboratory for the measurement of body length (in mm), carapace width (in mm) and wet weight (in mg). Prey characteristics Prey items present in the webs of spiders were also collected and identified to the level of order only. For prey collection webs of spiders located at two different heights (i.e., cm and cm) from ground were selected. Body size (length from head to the tip of the abdomen) of each insect was also recorded to determine the correlation (if any) between the predator size and prey length. Statistical analysis Kolmogorov-Smirnov test was used to check the normality of the data before statistical analyses. As there was no significant difference in data collected during two years so it was pooled together for statistical analyses. Size of insects trapped in the webs at each height (i.e., and ) was compared using Mann-Whitney U test, whereas the number of insects of each order trapped at each height was compared using Chi-square test. Principal component analysis (PCA) was performed to investigate whether studied spiders could be separated into different groups on the basis of capture area (web character) and body measure (carapace width). Capture area and carapace width was used for the PCA as significant difference was observed both in the capture area and carapace width among studied species (Table 1) and these parameter has been used for PCA in previous studies (Prenter et al., 2008). To determine the relationships between body measures (carapace width and total body length and body weight) and web characteristics (capture area, mesh size, number of radii and number of spiral) of spiders Pearson s correlation was used. Relationship between the predator size and prey length was also subjected to the regression analysis. All statistical analyses were performed using Minitab RESULTS Most of the specimens of L. decorata (93%) constructed inclined webs at the height that ranged from 40 cm to 123 cm from ground. Out of 115 studied spiders, 76% constructed webs above 80 cm height while remaining 24 % below this height. Spiders recorded from the upper webs ( cm) were significantly larger (D.F = 1, 48; F = 3.44; P<0.005) and heavier (D.F = 1, 48; F = 3.33; P<0.005) than the spiders recorded from the lower webs (40-80 cm). Time required to complete a web was 1.00 ± 0.21 hour. The average diameter of the web was 25 ± 6.8 cm. Although the mesh height of upper webs was higher than lower webs but the difference was non significant (D.F = 1, 48; F = 3.17; P>0.05). After constructing a web L. decorata were found waiting their prey at the centre of the web (hub). Different web characteristics and body measures recorded during the present study are given in the Table I. Results of Pearson s correlation showed that web characteristics of L. decorata were related differently to the various body measures. Web capture area of L. decorata increased

3 WEB CHARACTERISTICS OF LEUCAUGE DECORATA 263 significantly with carapace width (r = 0.54; P < 0.01) and body weight (r = 0.49; P < 0.01) but body length of L. decorata was not related to the capture area (P > 0.05).). Similarly the number of radii, number of spirals and mesh height were not correlated with any of the body measures (P > 0.05 for each case). Principal component analysis (PCA) indicated that collected spiders could not be separated clearly on the basis web carapace width and capture area (Fig. 1). Table I.- Web characteristics and body measures (means ± SE) of Leucauge decorata. Measurements cm cm P-value Web characteristics Web diameter (cm) 17 ± ± 6.8 * Capture area (mm 2 ) 6487± ± 789 * Number of radii 17 ± 4 17 ± 4 ns Number of spirals 21± 2 21± 3 ns Mesh height (mm) 1.6 ± ± 0.4 ns Body measures Carapace width (mm) 1.7± ± 0.63 * Body length (mm) 7.12 ± ± 1.6 ns Wet weight (mg) 25 ± ± 9.4 * Table II.- Prey orders Prey items caught by Leucauge decorata at two different heights. % prey at different heights cm cm Diptera Homoptera Lepidoptera Coleoptera 5 2 Orthoptera 9 7 Hymenoptera 4 5 Araneae 2 1 Unidentified prey 12 8 A total of 727 prey items of different insect orders were recorded from 115 webs. The main prey orders caught by L. decorata in the webs at different heights are given in the Table II. Most of the prey items recorded from the webs belonged to three orders i.e., Diptera, Homoptera and Lepidoptera. Number of prey items collected from upper webs were different from those collected from lower webs (Mann-Whitney Test, U = 36; P = 0.04). Significant difference was also observed when number of insects of different prey orders was compared at two different heights (X 2 = 14.34; d.f. =1; P<0.001). There was a significant positive correlation between carapace width of L. decorata and prey lengths taken by this species (r = 0.97; P < 0.001). DISCUSSION In the present study it was observed that most of the adult L. decorara constructed inclined webs at different heights in the rice fields. Inclined webs and variations in the web heights in different species of Leucauge have been reported in other studies as well (Eberhard, 1988; Henaut et al., 2006). Spiders in the high webs were larger and heavier than spiders in lower webs in the present study (Table I). The result is in accordance with the findings of Ibarra-Nu n ez et al. (2001) and Henaut et al. (2006). The difference in the web site selection among spiders might be for reducing intraspecific competition. Most of the observed spiders (76%) constructed webs above 80 cm. Larger and heavier orb web spiders seek the larger prey that can be caught at higher sites in and between plant canopies. Similarly a more open web structure with a larger capture area may be an adaptation to target the capture of larger prey (Uetz et al., 1978). High and large webs help to capture more insects especially larger size flying insects (Kajak, 1965; Nentwig, 1982, 1985). More insects were observed in the webs at cm height as compared to cm height. This result is contradictory to the findings of Henaut et al. (2006) who reported more insects at 60 cm height as compared to 160 cm height above ground in a similar study. In the present study most of the prey recorded from the webs belonged to Diptera, Homoptera and Lepidoptera. Hymenoptera, which was reported as dominant prey in the webs of Leucauge species in earlier studies (Chaco n and Eberhard, 1980; Ibarra- Nu n ez et al., 2001) was not recorded as a major prey item in the webs of L. decorata in the present study. Hymenoptera with good vision and maneuverability in flight may detect and evade the webs resulting in an under-representation of this prey orders in spider webs (Land, 1997).

4 264 H.M. TAHIR ET AL. 2 Second Component First Component Fig. 1. Result of principal component analysis. Please note that first component represents carapace width, while the second component represents the capture area. Results of Principal component analysis (PCA) showed that studied spiders could not be separated on the basis of carapace width and capture area. In the present study capture area increased significantly with carapace width. Similar results were reported by Heiling et al. (1998). A larger capture area results in high prey interception (Chacon and Eberhard, 1980). Mesh height did not relate to any of body measures in the study contrasting the results of Eberhard, 1988 and Heberstein et al. (2000). Some other studies also failed to find consistence relationship between mesh height and prey size (Heberstein and Heiling, 1998). It is concluded that for prey capture L. decorata construct webs of different sizes and at different heights but always maintain the basic web architecture (i.e., number of radii, number of spirals and mesh height). REFERENCES AIKEN, M. AND COYLE, F.A., Habitat distribution, life history and behavior of Tetragnatha spider species in the Great Smoky Mountains National Park. J. Arachnol., 28: ALCOCK, J Animal behavior. An evolutionary approach, Fifth Edition, Sinauer Associates Inc. Publishers, Sunderland, Massachusetts, pp CHACON, P. AND EBERHARD, W. G., Factors affecting numbers and kinds of prey caught in artificial spider webs, with consideration of how orb-webs trap prey. Bull. Br. Arachnol. Soc., 5: EBERHARD, W.G., Function and phylogeny of the spider webs. Ann. Rev. Ecol. Syst., 21: EBERHARD, W. G., Behavioral flexibility in orb-web construction: effect of supplies in different silk glands and spider size and weight. J. Arachnol., 16: EBERHARD, W.G., Effect of orb-web geometry on the prey interception and retention. In: Spiders, webs, behavior and evolution (ed. W. Shear), Stanford University Press, Stanford, pp HENAUT, Y., GARCIA-BALLINAS, J.A. AND ALAUZET, C., Variation in web construction in Leucauge venusta (Araneae, Tetragnathidae). J. Arachnol., 34: HEILING, A.M., HERBERSTEIN, E, M. AND SPITZER, G., Calculation of capture thread length in orb-webs:

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